This invention relates to control rods for a nuclear reactor and more particularly to cylindrical control rods containing a stack of neutron absorbing pellets.
It is common, particularly in nuclear reactors of the pressurized water type (PWR), to control the power output and power distribution in the reactor core with control rods insertable from the top of the reactor into the fuel assemblies. These control rods are typically hollow metal tubes containing stacked pellets of uniformly shaped neutron absorbing poison material, usually B.sub.4 C. In the core, the control rods reciprocate within control rod guide tubes, which provide an unobstructed path within the core while helping maintain the structural dimensions of the fuel assemblies.
The inner diameter of the guide tubes is usually chosen to be the maximum permitted by the fuel assembly lattice in order that the maximum possible diameter control rod can be inserted therein. It is desirable to maximize the diameter of the B.sub.4 C pellets in the control rod because the absorption effectiveness of the rods is very strongly dependent, particularly in thermal neutron reactors, on the surface area of the pellets. For this reason, and to promote heat transfer, there usually are narrow clearances between the B.sub.4 C pellets and the control rod clad, and between the control rod and its guide tube. The gap between the B.sub.4 C pellet and the clad must be large enough, however, to accommodate the swelling the pellets experience when they are irradiated while in the reactor core. It is very important that the swollen pellets not press too strongly against the clad wall because significant clad deformation can result in the control rod jamming in its guide tube. But if the gap is too large, chips that are dislodged from the B.sub.4 C pellets as a result of the control rod reciprocation will settle in the gap in the lower tip of the rod and quickly deform the clad as the pellets in the tip swell.
The effective lifetime of a control rod is determined by the average cumulative radiation exposure over the length of the rod, and by the peak exposure at any point in the rod. The average exposure limitation relates to the integrated destruction of the B-10 absorber isotope in the B.sub.4 C, which can eventually render even a fully inserted rod ineffective in controlling the reactor. The peak radiation limit relates, as described above, to the local clad strain and the possibility of the control rod jamming in its guide tube. A major problem in the design of control rods for use in power reactors has been the economically unfavorable fact that the control rod peak exposure limit is experienced in the end of the control rod nearest the core well before the rod average exposure limit is approached. This results from the high irradiation the leading tip experiences even when the entire rod is out of the core and in the withdrawn position. To make the reactor vessel longer so that the tip of the rod can be farther from the core when in the withdrawn position, is too costly.
One prior art solution is to fill the lower end of the control rod with a slug of silver-indium-cadmium (Ag-In-Cd), which does not experience the high rate of swelling characteristic of B.sub.4 C. Ag-In-Cd is much more expensive than B.sub.4 C, however, and has slightly lower neutron absorption strength.